FIG. 1 shows an axial section through a prior art bushing which consists of a unitary porcelain ceramic part 1 with a conical inside surface 2 and outside fins 3.
Cylindrical flanges 4 and 5 at the two ends allow the fitting of collars for fixing the bushing to the apparatus (not shown) to which it is connected.
The bushing is filled with a dielectric gas such as sulfur hexafluoride at a pressure of a few bars. One or more electrical conductors such as 6 pass coaxially through the bushing.
The porcelain may shatter in the event of an external shock; as a result, fragments may be projected violently outwards which could be extremely dangerous both for people and for nearby property.
In order to avoid the risk of the porcelain exploding, a lining 7 of strong material such as epoxy glass is placed adjacent the inside wall of the bushing, with the lining being capable of containing the pressure inside the volume V1 which it encloses, while the volume V2 between the lining 7 and the porcelain ought to be small.
The lining is not completely gas-tight, but in the event of the ceramic rupturing, it is sufficiently gas-tight for the gas to diffuse only slowly through the lining.
For reasons of cost, linings are sized accurately for each model of bushing.
Unfortunately, there are significant manufacturing tolerances in such bushings, about plus or minus one per cent, so that the end diameters D1 and D2 may vary considerably from one bushing to another, even for the same model of bushing.
It is therefore necessary to make linings which have the same lengths as the bushings but which have radial dimensions corresponding to the smallest values in the tolerance ranges of said diameters. The exact position of the lining inside the ceramic is then adjusted using spacers 8.
As a result, volume V2 can become significant (up to 30% of the total volume of the bushing in the worst case), and the gas contained within it could cause a dangerous explosion in the event of the ceramic rupturing.
It has been proposed to make linings which are exactly matched to each bushing, but that solution must be rejected for obvious economic reasons.
It has also been proposed to make the volume V2 between the lining and the ceramic completely gas-tight and to maintain within it a dielectric gas pressure which is lower than that in the volume V1.
Such a solution is clearly expensive since it is necessary to ensure that the volume is gas-tight, to fill the two volumes separately, and to provide two pressure monitoring systems.
It is an aim of the invention to produce a lined bushing at no increase in cost, said bushing being completely free from any risk of explosion.
The applicant has based the invention on the observation that the manufacturer can readily make bushings having a highly constant cone angle, even though there is considerable variation in the diameters of different bushings of the same model.